Microfocus X-ray device

ABSTRACT

In microfocus X-ray equipment for enlarging radiographic short-time recordings, a focussed electron beam for the production of X-radiation (16) impinges on the retarding material of a target (23). In this case, the retarding material in the focal spot (22) passes over into the liquid aggregate state due to the high thermal loading. For this reason, the equipment is operated in pulsed operation, wherein the position of the focal spot (22) on the target (23) is, when each loading occurs, displaced relative to the previous position. The retarding material is arranged in a retarding layer (32) on a carrier layer (33) and the electron beam (16) impinges on the retarding layer (32) oriented perpendicularly to the electron beam (16). A control interrupts the irradiation at the latest when the carrier layer (33) starts to melt.

DESCRIPTION

The invention relates to equipment of the kind known from U.S. Pat. No.4,344,013 (Ledley).

The usability of so-called direct and enlarging radiographic equipment,in particular in the fields of material testing and medicine, isdescribed more closely in the contribution "Entwicklung und Perspektivender medizinischen Vergrosserungsradiographie" by G. Reuther, H. -L.Kronholz and K. B. Huttenbrink in RADIOLOGE, volume 31 (1991), pages 403to 406. The function of such equipment is based on theradiation-geometric law, according to which a radiation source leads tohigh-contrast shadow images of high local resolution only when theradiation surface effective for imaging is very small by comparison withthe irradiated surface of the object to be imaged, because otherwiseeach point of the object would be irradiated at different angles, thusfrom different places of the radiation source, each object point onprojection into the image plane would result in shadow casts displacedrelative to one another and the result altogether would be a smudgedoutline of the object which is illustrated enlarged according to itsdistance from the image plane.

In spite of the improvement in the resolution achievable thereby, itemsof microfocus X-ray equipment have not been able to gain acceptance sowell in practice, in particular in medical diagnosis. This appears to betraced back above all to them being able to operate only with restrictedX-ray power, because the very narrow focussing of the electron beam ontothe retarding target results in a focus spot (focus) of very smalldiameter with correspondingly high energy density. This high specificloading rapidly leads to the target, which is usually irradiated at adirection of 10° to 45°, experiencing a change, which is disadvantageousfor the conversion of the impinging electron beam energy into X-rayenergy to be delivered, in its topography with rapid destruction of theretarding layer. Otherwise, the exposure time per X-ray recording wouldhave to be prolonged when X-rays of lower power were to be used, whichwould, however, contradict the demand for short exposure times in therange of tenths to hundredths of seconds in order to avoid anunnecessarily high beam loading and defocussing due to the movement ofthe object. However, the smaller the thermal focus spot is on the targetanode, the lower also becomes the electrical power which can be receivedby the small target area before it begins to melt. This behaviour thuscontradicts the requirement for higher density of the electron beamsimpinging on the target for higher power of the X-ray radiation.

An item of microfocus X-ray equipment, which operates already with atarget that has begun to melt, is known from the initially mentionedU.S. Pat. No. 4,344,013 (Ledley). In this equipment, the electron beamimpinges on an obliquely set target, so that the produced X-radiation issimilarly radiated away from the target at an angle. However, in thisequipment, it has not been taken into consideration that a rapidlyprogressing crater formation leads, even before complete burning-throughof the target, to the optical axis of the useful radiated X-rayradiation experiencing a shadowing by the crater rim that is swelling upand absorbs the X-ray radiation to a large extent. There results adiffuse X-ray light which cannot be regarded as emanating from apunctiform source. For that reason, equipment of that kind with anoblique setting of the target relative to the incident electron beam hasnot proved itself.

German preliminary published specification (DE-OS) 34 01 749 A1(Siemens) concerns X-ray equipment in which the electron beam isdeflected constantly and, for example, in meander shape on the retardingmaterial. However, the effective focus spot is thereby enlarged, as aresult of which the image sharpness suffers, as described above.

A transmission target, in which the retarding material is arranged on acarrier material, is known from German preliminary publishedspecification (DE-OS) 26 53 547 A1 (Koch and Sterzel). The avoidance ofa critical thermal loading, as occurs in microfocus equipment, is notdiscussed in this specification.

The invention therefore has the object of opening up further fields ofuse for microfocus radiography in that a radiation-geometricallyavailable X-ray radiation is produced in spite of minimised focal spotdiameter on the target.

Developments and refinements of the invention are claimed in thesubclaims.

An embodiment of the invention is illustrated in the drawings, in which:

FIG. 1 is a schematic longitudinal section through microfocus X-rayequipment,

FIG. 2 is a section through the target to enlarged scale,

FIG. 3 is the target according to FIG. 2 with a measurement of thetarget current,

FIG. 3A is the course of the target current in dependence on theduration of exposure,

FIG. 4 is a target with a retarding volume drawn in and

FIG. 4A is a carrier layer with carrier material dopings.

The microfocus X-ray equipment 1 consists of an evacuated housing 11 and12 of glass or non-ferromagnetic metal. The tube 12 has any desiredcross-section, which as a rule is round. Electrical feed wires 13 for acathode 14 in the form of a hair needle project through a rearward endface 11 of the tube 12 into the interior of the tube 12. The heatedcathode 14 acts as an electron source, from the radiation of which asmall divergent electron beam 16 is masked out by means of a cap-shapedgrid 15. The beam 16 passes through the central opening of a perforateddisc anode 17 and in that case experiences a focussing to a virtualfocal spot 18. The beam 16, which thereafter widens out again, passesthrough the cross-sectional zone of a deflecting coil 19 arrangedexternally of the tube 12 and is focussed in the magnetic gap 20 of anadjoining focussing coil 21. The focussing coil 21 as electromagneticlens forms a reduced image of the virtual focal spot 18 as a focal spot22 on a transmission target 23, which is disposed in the exit opening 24of the tube 12. The focussing coil 21 produces a focal spot 22 ofextremely small area in the order of magnitude of typically 0.5 to 100micrometres. The target 23 consists of a thin retarding layer 32 of ametal of high atomic number in the periodic system of elements, such astungsten, gold, copper or molybdenum, and a carrier layer 33, preferablyof aluminium or beryllium, which absorbs X-rays poorly, but is thermallyhighly conductive. In consequence of the retarding effect of the targetmaterial, the impinging electrons of the beam 16 initiate theX-radiation 25. A part of the X-ray radiation 25 penetrates the target23 with the beam direction 28, which coincides with the beam axis 10 ofthe electron beam 16, and leaves the tube 12 in the direction towards asample 26 as a divergent X-ray beam 25. By reason of the geometricradiation law, the structure of the sample 26, insofar as it is more orless impermeable by the X-rays 25, is projected correspondingly enlargedin the image plane 29 as shadow outline onto a film arranged at agreater spacing behind the sample 26 parallel to the transmission target23 and thus perpendicularly to the beam direction 28.

A suction plant 37 for maintenance of the vacuum in the tube 12 and forextraction of vaporous material traces of the cathode 14 to be combustedacts at the same time to keep the interior space of the tube 12 clean ofmolten material particles from the focal spot hole 31 in the target 23.

The particularly high yield of X-rays 25 results from the excitedretarding volume 40 of extremely small area (FIG. 4) in the transmissiontarget 23. The high power density, thus the high physical loading perunit area by the microfocussed electron beam 16, leads to the burning ofa focal spot hole 31 into the target 23, so that the remaining targetmaterial and thereby its radiation-attenuating inherent absorptionreduces continuously in the departure direction 28 of the X-rays 25. Theretarding layer 32 is melted away in targeted manner by the impingingelectron beam 16, which with respect to its aggregate state represents adynamically changing X-ray source.

When the retarding material is borne as a thin layer, possibly oftungsten, on a carrier layer 33, which is thick by comparison therewithand of thermally highly conductive material, such as beryllium oraluminium, then it is hardly avoidable, but also uncritical, that at thebase of the hole 31 in the retarding layer 32 the carrier layer 33 lyingtherebehind in radiation direction 28 is also ultimately melted by themicrofocussed electron beam 16. Then, however, the radiation of thetarget 23 must be terminated at this position, thus the recording beended in the application of this X-ray equipment 1, because the loadingof the carrier layer 33 by electron beams 16 leads only to a very softX-radiation 25 and thus to hardly usable diffuse shadow images of thesample 26, which is to be transilluminated, in the image plane 29.

For the next X-ray shadow image to be recorded, the very briefirradiation of the transmission target 23 is again affected by amicrofocussed electron beam 16, for which purpose the cathode 14 isagain operated for only a short time and/or the beam 16 is freed onlybriefly by way of a pivotable aperture stop, which is not illustrated inthe drawing, or the beam 16 is pivoted by way of a corresponding drivecontrol of the deflecting coil 19 briefly from a non-functional waitingdirection into the instrument--and effective--axis 10 of the beamdirection 28. However, at the transmission target 23, a place at which ahole 31 has been presumably burnt in may not be irradiated again,because otherwise the carrier layer 33 would soon or even immediately bemelted instead of the retarding layer 32 of retarding material. For thatreason, the displacement control 34 is provided, which, by theafore-described beam deflection by means of the deflecting coil 19 fromthe instrument axis 10 and/or through redisposition of the target 23relative to the instrument axis 10, ensures that successive focal spots22 are caused only along a path extending in meander or spiral shape. Itis thereby ensured that only unused regions of the target 23 are loadedone after the other and thus a destruction of the carrier layer 33 withinitiation of only little useful, and moreover low-energy, X-radiationis avoided. The target 23 is thus so loaded in transmitted lightoperation by the perpendicular charging by electrons until an aggregrateconversion into the molten phase sets in.

For illustration of the redisposition of the target 23 relative to thetube 12 or its axis 10, a positioning motor 35 is disposed in the tube,illustrated graphically in the drawing. Instead thereof, the target 23together with the positioning motor 35 can basically also be retained invacuum-tight manner at the end face in front of the exit 24 of the tube12 or a linkage from an external arrangement of the positioning motor 35engages through the wall at a rotary or sliding mount 36 for the targetin the interior of the tube 12.

As has been explained in the preceding, the redisposition of the target23 must take place whenever the electron beam 16 has burnt the microhole31 so deeply into the retarding layer 32 that it reaches the carrierlayer 33.

A simple procedure for ascertaining this instant consists in that aftera short exposure time, which can be estimated with reference to thepower or even more easily can be determinable empirically, in the orderof magnitude of milliseconds or microseconds, the focal spot productionon the target 23 is to be terminated, for which purpose the electronbeam can be switched off, masked off or pivoted out of the target range,as already described in the preceding. This procedure does not, however,take the individual state of the microhole 31 into consideration. It canthus well be the case that the carrier layer 33 in this procedure isalready irradiated or that the microhole 31 on the other hand has notyet reached the boundary between the retarding layer 32 and the carrierlayer 33.

A substantially more accurate method for ascertaining the instant t_(a)at which the retarding layer 32 is molten through and the electronsimpinge on the carrier layer 33, is measurement, which is reproduced inFIG. 3, of the target current I. When the target current I is measured,as illustrated in FIG. 3, as a function of the exposure time t, thenthis has the course illustrated in FIG. 3A. At the instant t_(a), asudden increase in the target current takes place. The instant t_(a) isthat instant at which the electron beam has penetrated the retardinglayer 32 and the microhole 31 reaches to the carrier layer 33. Bymeasurement of the target current I, a command for deflection of theelectron beam 16 can thus be obtained very easily by the control. Inthis case, all local characteristics of the retarding layer 32 and thecarrier layer 33 are automatically taken into consideration.

When an electron accelerated in a high-voltage field penetrates into thesurface of matter, it experiences a sequence of elastic impacts, duringeach of which it loses a part of its kinetic energy which converts intoradiation, in reaction with the matter. A part of this radiationconsists of X-radiation. During the sequence of elastic impacts, theelectron passes within the target material through a retarding volume 40(FIG. 4), the extent of which is determined primarily by the atomicnumber Z of the target material, the energy E_(o) of the electrons andby the electron beam diameter t.

The X-radiation rises within the described retarding volume 40. Theextent of the radiation source is thus determined by the magnitude ofthe retarding volume 40. Even if an electron beam diameter d tending to"zero" is assumed, a finite retarding volume 40 remains in consequenceof the spreading of the electrons. Thus, a minimum radiation source sizedetermined substantially by E_(o) and Z can in principle not be fallenbelow.

If now a further reduction in size of the radiation source is to beachieved, target material dopings 41 (FIG. 4A) must be introduced intothe carrier material, the volumes of which are each significantlysmaller than the afore-described retarding volume 40 of the electrodesin a coherent target material.

The usable X-radiation arises only in target material of higher atomicnumber. The electrons, which have penetrated from the target materialdopings 41 into the carrier material of lower atomic number, do notcontribute to the usable X-radiation, as also the electrons penetratingdirectly into the carrier material beside the dopings 41 do notcontribute substantially to the usable radiation.

Since fewer X-ray photons per unit time for the same electron beamdensity thus arise in the small doping volumes according to FIG. 4A thanin the greater retarding volumes 40 in a retarding layer 32 (FIG. 2),the electron beam density (current) must be increased. Although thisleads to a rapid melting-away of the target material dopings 41 andtheir carrier material surrounding, the X radiation arising during themelting process can, however, also be utilised. For the next X-rayrecording, the electron beam 16 is deflected in known manner to a stillunused doping place 41 and so forth. The dopings 41 can, for example, bearranged in a defined raster.

LIST OF REFERENCE SYMBOLS

1 microfocus X-ray equipment

10 instrument and beam axis

11 end face

12 tube

13 feed wires

14 cathode

15 grid

16 electron beam

17 perforated disc

18 virtual focal spot

19 deflecting coil

20 magnetic gap

21 focussing coil

22 focal spot

23 transmission target

24 exit opening

25 X-radiation

26 sample

28 radiation direction of the X-rays

29 image plane

31 microhole

32 retarding layer

33 carrier layer

34 displacement control

35 positioning motor

36 rotary or slide mounting

37 suction plant

40 retarding volume

41 dopings

I claim:
 1. Microfocus X-ray equipment comprising generating means forgenerating a focused electron beam for impinging perpendicularly on atarget for the purpose of production of X-ray radiation, the targethaving a carrier layer and a retarding layer at a side of the carrierlayer facing the beam and the retarding layer comprising a retardingmaterial which changes at the focal spot of the beam into at least theliquid aggregate state under the thermal loading of the beam, displacingmeans for displacing the focal spot on the target relative to theprevious spot position with each said thermal loading, and control meansfor interrupting the beam at the latest when the carrier layer starts tomelt and for determining the instant of said start of melting of thecarrier layer by measurement of the target current.
 2. Equipmentaccording to claim 1, wherein the retarding material is present in theform of dopings in the carrier layer.